Despite numerous studies over the years, energy transfer processes are still interesting from both a fundamental and an applied point of view, as transfer processes in pairs of ions and from the host to luminescent ions are crucial for development of innovative phosphors, scintillators and materials for solar cells. Here I will present results on these processes, concentrating on the ion pairs Tb3+-Eu3+ in eulytite phosphate hosts.

Eulytite double phosphates A3M(PO4)3 (A=Ca, Sr, Ba; M=La-Lu, Y, Bi) are well known to be isotypic with the mineral eulytine (Bi4Si3O12). The emission spectrum of neat Sr3Tb(PO4)3 upon excitation at 337 nm in the levels above 5D3 is dominated by 5D4 emission; no significant emission from 5D3 is observed due to efficient cross relaxation involving the Tb3+ levels. On the other hand, the emission spectrum of the same host containing 10 mol% Eu3+ upon excitation at the same wavelength (in the Tb3+ levels) is dominated by strong emission bands from the 5D0 level of Eu3+. This indicates that Tb3+®Eu3+ energy transfer is clearly present. The excitation spectrum of the Eu3+5D0 emission is dominated by strong Tb3+ bands extending in the UV region.

The presence of 10% Eu3+ in Sr3Tb(PO4)3 very effectively shortens the 5D4 decay time from 2.68 to 0.20 ms. The decay curve is not far from exponential, indicating that the energy transfer to Eu3+ is accompanied by fast energy migration. The efficiency of the energy transfer from 5D4 amounts to 0.93, and the critical distance for the transfer from the 5D4 level of Tb3+ is about 7 ?.

The results obtained for the eulytite hosts will be compared with the ones recently obtained for double silicate hosts containing the same Tb3+ and Eu3+ ions.